It has previously been proposed to manufacture core structures of soft magnetic properties which contain from between 95 to 99.5% carbonyl iron, and the rest an organic binder (by weight). Carbonyl iron is comparatively expensive so that magnets made from this material are also comparatively expensive. Additionally, since the filler or active component is a very high proportion of the overall mass, the structure is difficult to manufacture since it does not flow readily. Especially complex shapes, therefore, are difficult to make since the mass will not flow easily and uniformly in the die therefore. The mass must be compacted with extremely high pressures, in the order of from between 5000 to 18000 bar. The mechanical strength of the structures made by this mass is low, and the articles are brittle. Further, it is difficult to match the magnetic properties of cores made from such compacted masses to desired technical requirements.
It has also been proposed to make magnetic cores of soft magnetic sintered ferrites. These materials have a lower magnetic saturation polarization, a lower mechanical strength, and a higher dependence on temperature of the magnetic characteristics thereof. The possibility of providing different structural shapes to magnets of this type are very limited, and the tolerances of their dimensions are high. They are difficult to be machined after sintering.
Cores have also been made of transformer iron or transformer laminar sheets. Laminated cores can be used only up to frequencies of about 1 kHz due to eddy current losses. The structural shapes which can be obtained by such cores also are limited, and the cores have to be stacked and connected together, which is a comparatively expensive manufacturing operation. Some sheets which have a thickness of only 0.03 mm or less, and made of nickel-iron alloys, can be used for frequencies up to 100 kHz and have higher permeabilities; these sheets, however, are difficult to handle, to machine, and are expensive.